One major problem that makes a viral infection so hard to cure is that they mutate so rapidly. It also hard to find drugs that will work. Tamiflu is an antiviral that acts against an enzyme that the virus needs to break out of the host cell and spread but many viruses are so simple that there are very few ways to stop them.
Viruses can mutate rapidly, which makes it difficult for the immune system to identify the virus and produce the antibodies needed.
Viruses are nonliving and that fact makes it hard for us to come up with antivirals.